United States federal legislation on trucks requires that sound level meter readings be taken during a transient "drive-by" test (CFR40, Part 202). The test condition requires driving a vehicle past a microphone and accelerating from two-thirds of maximum rpm to maximum rpm at full throttle. This test condition results in primarily an inertial loading (F=MA, where F=force=torque produced by engine multiplied by gear ratio, M=mass of truck, and A=acceleration of truck) of the engine. Frictional loading and vehicle drag are minimal because the test specifies a maximum vehicle speed of 35 miles per hour. The speed limitation also minimizes noises produced by the tires on the road surface.
Since the early 1970's in order to do research on mufflers to minimize the noise contribution due to the exhaust, sound testing has been commonly done with a dynamo-meter loaded engine. A prime requirement has been to isolate all engine and/or vehicle noise to a level well below the exhaust noise from the muffler to obtain sufficiently sensitive data.
In 1972, for example, U.S. Pat. No. 3,695,098 disclosed an underground test cell and method of its use. To test a muffler in accordance with that disclosure, an engine connected to a dynamometer is operated in the underground test cell with exhaust gases being directed through the roof of the underground test cell to a muffler mounted on a truck on the roof of the underground test cell. In this way, all sound sources are isolated in the test cell so that the muffler is the significant sound source outside the cell and only the truck body affects the radiation pattern of the sound from the muffler.
Another method which has been used is to partially shield the engine of a truck with a soundproof barrier of lead and fiberglass. The lead and fiberglass is generally placed over the entire hood and draped down along the front and sides of the truck. The draped portion extends near the ground, but does not contact the ground. The fan is disconnected to eliminate its noise. The truck is driven according to the speed and acceleration requirements for the test (ref. CRF 40, Part 202). The problem with this method is that it requires significant care and maintenance. The engine cannot be run for very long periods since the fan is not used and overheating can occur. The lead and fiberglass is difficult to work with since the hood needs to be opened to service the motor and the movement of the truck can displace the material. In addition, this method does not provide for sufficient sound source isolation in order to accurately measure the sound level of mufflers.